1. Field of the Invention
This invention provides a system capable of gradually transitioning from stereo to ably simulated surround sound, and vice versa using front speakers.
2. General Background and State of the Art
With multi-channel audio devices, a listener has an option to hear the audio in a surround sound mode. For example, home theater systems are generally connected to front (left, right and center) and rear speakers to generate the surround sound in a listening room. For surround sound effect, rear speakers are typically needed in order to generate sound from the side or rear of the listener. If wires are not already installed in the listening area, wires need to be installed between the amplifier and the rear speakers. Installing wires for rear speakers, however, can be inconvenient and sometimes the wires can show so that they may be esthetically unpleasing.
Some have tried using just two front speakers to create a sound pressure field to simulate surround sound to eliminate the need for the rear speakers. One way to accomplish this is to filter each of the multiple left signals and the multiple right signals from the multi-channel device with appropriate ipsilateral and contralateral positional filters, to produce filtered left and right audio channels. These ipsilateral and contralateral positional filters are derived from a head related transfer function (HRTF) measured impulse response. These filters may give the listener the impression that the sound from the two front speakers are originating from virtual front and surround or rear speakers. All the left filter output signals are then summed together to make a left composite channel, and all the right channel signals are summed to make a composite right channel. Such a system also needs to cancel the cross talk associated with the left and right loudspeakers. This may be accomplished through filtering the composite left and right channels with the inverse HRTF transformation associated with the real loudspeaker positions. To calculate the inverse HRTF, measurements of the shape and size of the listener's ears (the “pinnae”) and head may need to be taken. This can be a complicated process that takes time and adds cost so it may be not be a practical way to simulate surround sound effect. An average inverse HRTF may be used, but without the actual measurements of the pinnae, the quality of the simulated surround sound effect may be poor.
Another way to simulate surround sound is to use dipole and monopole pressure fields derived from point sources without having to calculate the inverse HRTF. This method models the listener's ears as two points separated by a distance 2a, where a represents the listeners head radius so that the head diffraction may not be taken into account in calculating the dipole and the monopole. However, this methodology does not include an option to hear pure stereo or a way for a listener to transition from stereo to surround sound effect, or from monophonic sound field to stereo. Listener preferences may vary as to the amount of surround effect, and the configuration of the room can affect the sound pressure field produced by the two physical front speakers. Therefore, a need exists for a system that provides an option for a listener to control the transitions from the full surround effect to stereo, or even to monaural.